1. Field of the Invention
The present invention relates to a DC source system and its operation method and, more particularly, to a DC source system in which a solar cell and a DC source are connected in parallel with each other, and its operation method.
2. Related Background Art
In recent years, global warming, exhaustion of fossil fuels, radioactive pollution caused by nuclear power plant failures and radioactive waste, and the like pose serious problems, and people are increasingly showing interest in global environment and energy problems. Under such situation, a solar cell is expected as an inexhaustible, clean energy source. Systems that utilize solar cells have a variety of scales ranging from several watts to several thousands of watts, and various kinds of systems are available. For example, a system that directly uses generated electricity as power, a system that stores generated electricity in a battery, a system that utilizes a solar cell in cooperation with a commercial source, and the like are known. Of these systems, as the system that utilizes a solar cell as a source in cooperation with the commercial source, a power system which converts a DC voltage output from the solar cell into an AC voltage using an inverter and performs reverse load flow with respect to the commercial source has been proposed. However, the inverter is expensive, and such system is not yet popular. Also, a method which utilizes a DC voltage output from a solar cell in an inverter air conditioner via a DC/DC converter (Japanese Laid-Open Patent Application No. 4-174262), and a system in which a solar cell is DC cascade connected to a DC source from a commercial system via a reverse-flow prevention diode so as to be connected to a DC load (e.g., an inverter air conditioner) including an inverter (Japanese Laid-Open Patent Application No. 4-109863) have been proposed.
However, when the DC voltage output from the solar cell is supplied to a load via the DC/DC converter, the cost increases, as described above.
When the solar cell is DC cascade connected via the reverse-flow prevention diode, the DC voltage output from the solar cell rises when the load decreases or stops, and the open voltage of the solar cell or a voltage approximate to that voltage may be applied to the inverter.
When such voltage is applied to the inverter, if it is higher than the withstand voltage of the inverter, the inverter may be destroyed. For this reason, the open voltage of the solar cell must be set to be equal to or lower than the withstand voltage of the inverter.
Therefore, when the solar cell is simply operated at an optimum operation voltage, a problem may arise depending on the load state.